1. Field of the Invention
The present invention relates to an optical recording medium, a method of producing the same, and an optical recording and reproduction device, more particularly an optical recording medium used in the near field and preventing a decrease of intensity of reproduced signals accompanying fluctuation of the distance between the optical recording medium and optical system and preventing damage due to collision with an optical system and a method of producing the same and an optical recording and reproduction device including the optical recording medium.
2. Description of the Related Art
Up to now, a hard disk or other magnetic recording medium has been used in a state with a head for recording and reproduction brought into extremely close proximity to the disk or other medium for the purpose of obtaining good signal characteristics. As opposed to this, a phase change type optical disk, magneto-optical disk, or other optical recording medium has been used in a state with the optical system or head for recording and reproduction separated from the recording medium by a predetermined distance.
However, in recent years, in the devices used for optical recording media, the system of bringing the optical system or head into close proximity, for example, 200 nm with the disk (near field) has begun to be employed for the purpose of increasing a numerical aperture (NA) of the optical system and thereby increasing a recording density of the disk.
As an optical recording medium device used at the near field, for example, there are an optical hard disk structured with a lens mounted on a slider, an optical disk device with a lens made movable by an electromagnetic actuator, etc. In these devices, light for recording and reproduction is focused on the recording medium by an optical system comprised of a plurality of lenses including at least an objective lens and a solid immersion lens (SIL). Due to this, an NA of over 1 has been obtained.
FIG. 1 is a schematic view of a hard disk. The disk 1 is structured with a recording layer 3 and a lubrication film 4 stacked on a substrate 2. A recording and reproducing head 5 for changing the magnetization of the recording layer 3 is mounted on a slider 6 and movable in the direction of the disk plane. The lubrication film 4 is provided for preventing abrasion of the head 5 and the disk 1. The lubrication film 4 can be formed, for example, by coating a fluorine compound. In the case of an optical disk, consideration of optical conditions is required for a layer formed on a recording layer, but the lubrication film 4 of a hard disk does not require consideration of optical conditions as required for the optical disk. Therefore, it can be relatively easily formed.
FIG. 2 is a schematic view of a conventional optical disk with a large distance between an optical system or head and the disk, for example, a phase change type optical disk or a magneto-optical disk. The optical disk of FIG. 2 is structured with a dielectric protective layer 12, a recording layer 13, a dielectric protective layer 14, a reflective film 15, and a resinous protective layer 16 sequentially stacked on a substrate 11. In the case of a phase change type optical disk, a material changing in phase by focusing of light is used for the recording layer 13. In the case of a magneto-optical disk, a material changing in magnetization state using focusing of light is used for the recording layer 13.
In the optical disk of FIG. 2, both surfaces of the recording layer 13 are protected by the dielectric protective layers 12, 14. These surfaces are further protected by the substrate 11 or the resinous protective layer 16. A distance between a lens 17 and the disk is much larger than that of a hard disk. A film for dealing with friction or collision between the lens 17 or head and the disk is usually preferable, but not necessary.
FIG. 3 is a cross-sectional view of an optical disk used in the near field. It is structured by a reflective layer 22, a second dielectric layer 23, a recording layer 24, and a first dielectric layer 25 sequentially stacked on a substrate 21. In the case of the optical disk shown in FIG. 2, light is focused from the side at which the transparent substrate 11 is formed. On the other hand, in the case of the optical disk for near-field use shown in FIG. 3, light is focused from the side at which the first dielectric layer 25 is formed. Due to this, the increase in the coma along with an increase in the NA is moderated.
In the optical disk of FIG. 3, the four layers of the first dielectric layer 25, the recording layer 24, the second dielectric layer 23, and the reflective layer 22 are optimized in design for obtaining good signal characteristics for light striking the disk surface perpendicularly.
On the other hand, in the case of a near-field optical disk device having a short distance between the head and the disk as described above, the risk of collision of a head or a lens and the rest of the optical system with the disk becomes extremely high. However, it is very difficult to uniformly coat a lubricating substance such as used for the lubrication film 4 of the hard disk on the surface of an optical disk to form a thin film satisfying the optical conditions. Also, in the case of a near-field configuration, the fluorine material used for the lubrication film 4 of a hard disk cannot be used because the refractive index is too low. There are few other suitable materials.
When an antireflection coating (AR coating) is provided on the surface of the lens, once the AR coating at the lens side is damaged due to collision, the recording and reproduction are influenced by the damage at all times. That is, a change of the optical characteristics of the entire device is caused. However, it is difficult to find a suitable coating material resistant to damage by collision as the material for the AR coating.
According to a film configuration of the above conventional near-field optical disk, there is a problem that light of a component of a high NA, that is, the light having a large incident angle with respect to the recording layer 24 is hard to reach the recording layer 24. This is caused by a large reflectance at the surface of the first dielectric layer 25 with respect to the light of the component of the high NA. Also, in a state where a space (below, referenced as t) between the lens and the surface of the disk is extremely small, when t fluctuates slightly, along with this, the reflectance of the surface of the first dielectric layer 25 greatly changes.
In a phase change type optical disk of the conventional configuration shown in FIG. 3, the reflectance of the surface of the first dielectric layer was calculated with an air layer between the lens and the surface of the disk regarded as a thin layer having a refractive index n of 1 and a thickness of t when light from an optical part (lens) of n=1.8 strikes the disk with various incident angles (an increase of the incident angle corresponds to an increase of NA). The calculation was performed when the incident angle is 0xc2x0, 100xc2x0, 20xc2x0, 30xc2x0, 40xc2x0 or 50xc2x0. The results of calculation are shown in FIG. 4.
As shown in FIG. 4, along with an Increase of the incident angle, the reflectance of the first dielectric layer becomes higher. Also, when the thickness t of the air layer changes to about 0 to 100 nm, the reflectance of the first dielectric layer drastically rises. In the actual near-field optical disk device, the space t between the lens and the surface of the disk is usually about 50 to 200 nm. It overlaps the region where the reflectance of the first dielectric layer greatly fluctuates. Therefore, when t changes slightly due to rotation of the disk etc., a distribution of energy of light in a disk plane easily fluctuates and it becomes hard to stabilize the reproduced signal level.
Further, in the above conventional near-field optical disk, when the space t between the lens and the surface of the disk slightly fluctuates, there is a problem that the reproduced signal level greatly changes along with a change of the optical characteristics.
In the phase change type optical disk of the conventional configuration shown in FIG. 3, the intensity of the reproduced signal was calculated with an air layer between the lens and the surface of the disk regarded as a thin layer having a refractive index n=1 and a thickness of t when light from an optical part (lens) of n=1.8 strikes the disk in various incident angles. The calculation was performed in the case of the thickness t of the air layer of 0 nm, 50 nm, 100 nm, 150 nm. The results of the calculation, that is, dependency of the reproduced signal on spatial frequency are shown in FIG. 5. The reproduced signal level of the ordinate is a value equivalent to modulation transfer function (MTF) expressing the normalized intensity of the reproduced signal.
As shown in FIG. 5, when increasing the thickness t of the air layer, the reproduced signal level drastically decreases. Therefore, it is necessary to reduce the distance t between the lens and the disk for increasing the reproduced signal level. When a recording and reproducing system where the lens and the disk are brought into extremely close proximity is used, the recording layer is easily damaged due to collision of the lens with the surface of the disk.
However, according to the conventional configuration shown in FIG. 3, the first dielectric layer 25, that is, a thin layer of for example ZnSxe2x80x94SiO2 or SiN, is formed on the outermost layer of the disk. Therefore, when the lens and the disk collide, the first dielectric layer 25 and the recording layer 24 below the layer 25 are readily damaged.
Also, since the lens and the disk are brought into close proximity than ever before, when there is a protruding defect on the surface of the disk, the protruding part may easily contact the lens and damage the surface of the lens. If the disk surface is polished for improving the surface conditions to prevent this, then the possibility of breaking the recording layer 24 by polishing becomes high since the first dielectric layer 25 formed on the recording layer 24 is extremely thin.
Further, when only the extremely thin first dielectric layer 25 is formed on the recording layer 24, local light absorption easily occurs at the surface of the disk. If ablation occurs due to laser light at the time of recording or reproduction, the disk will be damaged and the lens contaminated by the deposition of the disk material on the lens surface.
An object of the present invention is to provide an optical recording medium used in the near field having a high coupling efficiency of focused light and able to obtain a stabilized intensity of the reproduced signal and an optical recording and reproduction device Including the same.
Another object of the present invention is to provide an optical recording medium used in the near field and preventing decrease in Intensity of the reproduced signal along with fluctuation of distance between the optical recording medium and an optical system and an optical recording and reproduction device including the same.
Still another object of the present invention is to provide an optical recording medium used in the near field and preventing damage due to collision with an optical system and an optical recording and reproduction device including the medium.
Another object of the present invention is to provide a method of producing an optical recording medium able to produce the above optical recording medium by a simplified process.
According to a first aspect of the present invention, there is provided an optical recording medium, comprising a substrate, a recording layer formed on the substrate, a first protective layer formed on the recording layer, and an antireflection multilayer film, designed to be used in a near field condition, formed on the first protective layer for preventing reflection of light at the surface of the first protective layer and having predetermined optical characteristics for moderating a change of a reproduced signal intensity responsive to fluctuation of the near field condition.
Preferably, the predetermined optical characteristics are determined by a refractive index and thickness of each layer comprising the antireflection multilayer film.
Preferably, the antireflection multilayer film comprises at least two layers and the outermost layer of the antireflection multilayer film has a surface hardness able to be polished. Preferably, the outermost layer of the antireflection multilayer film comprises a silicon oxide layer. Preferably, the thickness of the silicon oxide layer is about 100 nm or more. Preferably, the antireflection multilayer film comprise a plurality of silicon oxide layers.
Preferably, a surface of the antireflection multilayer film is flattened and has no protruding defects. Preferably, there is a reflective layer comprised of metal or semimetal reflecting light formed between the substrate and the recording layer. Preferably, there is further a second protective layer formed between the substrate and the recording layer.
Preferably, the recording layer comprises a material undergoing a phase change and changing in complex index of refraction by focusing of light. Alternatively, the recording layer comprises a material changing in a magnetization state by focusing of light and enabling detection of the change as a change of a polarization state. Alternatively, the recording layer comprises an organic dye material changing in complex index of refraction or shape with respect to a wavelength of the reproduction light by focusing of light. Alternatively, the recording layer preferably comprises pits formed on a surface of the substrate and the optical recording medium is a read-only memory.
Due to this, it is made possible to decrease the reflectance and increase the coupling efficiency even with respect to light not striking the surface of the optical recording medium perpendicularly. In other words, it is possible to improve the MTF with respect to light of a wide range of incident angles. Therefore, it is possible to obtain a high MTF with respect to light from an optical system of a high NA.
Also, while the MTF decreases when increasing the distance between the optical system and optical recording medium, according to the optical recording medium of the present invention, it is possible to suppress a decrease of the MTF. Due to this, it is made possible to increase the distance between the optical system and the optical recording medium to a certain extent. Therefore, it is possible to prevent contact or collision of the optical system with the optical recording medium.
Further, according to the optical recording medium of the present invention, since the antireflection multilayer film is formed on the surface of the optical recording medium, it is not necessary to provide an antireflection coating (AR coating) on a surface of a lens of the optical system. Therefore, it is possible to prevent damage of the antireflection coating on the surface of the lens due to collision with the optical recording medium etc. and the constant effect of the damage at the time of recording and reproduction.
According to a second aspect of the present invention, there is provided a method of producing an optical recording medium used in a near field condition, comprising the steps of forming a recording layer on a substrate, forming a first dielectric layer on the recording layer, forming an antireflection multilayer film, designed to be positioned at a near field distance from an optical system, on the first protective layer for preventing reflection of the light at the surface of the first protective layer and having predetermined optical characteristics for moderating a change of a reproduced signal intensity along with fluctuation of a distance between the antireflection multilayer film and the optical system, and flattening a surface of the antireflection multilayer film by polishing.
Preferably, the steps of forming the recording layer, first protective layer, and antireflection multilayer film are steps of forming layers by sputtering.
Preferably, the polishing step comprises flying tape polishing (FTP) for removing protruding portions.
Due to this, it is possible to form an antireflection multilayer film having a smooth surface by a simplified process. By forming each layer by sputtering, it is possible to form the layers sequentially and each layer can be formed in a uniform thickness. When the antireflection multilayer film is polished by the FTP process, it is possible to polish the film in a short time. As described above, according to the method of producing an optical recording medium of the present invention, it is possible to form a near-field optical recording medium giving a high intensity of reproduced signals and resistant to damage by a simplified process.
According to a third aspect of the present invention, there is provided an optical recording and reproduction device comprising a light source, an optical recording medium, and an optical system focusing light from the light source to the optical recording medium at a near field distance from the optical recording medium, the medium comprising a substrate, a recording layer formed on the substrate, a first protective layer formed on the recording layer, and an antireflection multilayer film formed on the first protective layer for preventing reflection of the light at a surface of the first protective layer and having predetermined optical characteristics moderating a change of reproduced signal intensity along with fluctuation of the distance, and said device performing at least one of recording and reproduction of information by focusing light from the optical system to the recording layer via a side of the optical recording medium at which the antireflection multilayer film is formed.
Preferably, the predetermined optical characteristics are determined by a refractive index and thickness of each layer comprised of the antireflection multilayer film.
Preferably, the antireflection multilayer film comprise at least two layers and the outermost layer of the antireflection multilayer film has a surface hardness able to be polished. Preferably, the outermost layer of the antireflection multilayer film comprises a silicon oxide layer. Preferably, the thickness of the silicon oxide layer is about 100 nm or more. Preferably, the antireflection multilayer film comprise a plurality of silicon oxide layers.
Preferably, a surface of the antireflection multilayer film is flattened and has no protruding defect liable to damage the optical system.
Preferably, a reflective layer comprised of a metal or semimetal is formed between the substrate and the recording layer. Preferably, a second protective layer is formed between the substrate and the recording layer.
Preferably, the recording layer comprises a material undergoing a phase change and changing in a complex index of refraction by the focusing of light. Alternatively, the recording layer comprises a material changing in a magnetization state by the focusing of light and enabling detection of the change as a change of a polarization state. Alternatively, the recording layer comprises an organic dye material changing in complex index of refraction or shape with respect to a wavelength of the reproduction light by the focusing of light. Alternatively, the recording layer preferably comprises pits formed on a surface of the substrate and the optical recording medium is a read-only memory.
Due to this, it is made possible to decrease the reflectance and increase the coupling efficiency even with respect to light not striking the surface of the optical recording medium perpendicularly. In other words, it is possible to improve the MTF with respect to light of a wide range of incident angles. Therefore, it is possible to obtain a high MTF with respect to light from an optical system of a high NA.
Also, while the MTF decreases when increasing the distance between the optical system and optical recording medium, according to the optical recording and reproduction device of the present invention, it is possible to suppress a decrease of the MTF. Due to this, it is made possible to increase the distance between the optical system and the optical recording medium to a certain extent. Therefore, it is possible to prevent contact or collision of the optical system with the optical recording medium.
Further, according to the optical recording and reproduction device of the present invention, since the antireflection multilayer film is formed on the surface of the optical recording medium, it is not necessary to provide an antireflection coating (AR coating) on a surface of a lens of the optical system. Therefore, it is possible to prevent damage of the antireflection coating on the surface of the lens due to collision with the optical recording medium etc. and constant effect of the damage at the time of recording and reproduction.